read some bytes from a file with promises

read-file-promised.js

'use strict'

const fs = require('fs')

// promise-style fs subset (at bottom of this file)
const fsp = requirePromised_fs()

// read 12 bytes of this file, print them, or print error if that happens
readFile(__filename, 12)
.then(s => console.log('read:', s.toString()))
.catch(e => console.log('error:', e))

// return a promise to a Buffer len bytes long of first bytes of fileName
function readFile(fileName, len) {
  // allocate a buffer
  const buff = new Buffer(len)

  // get a promise on the open file
  const Δfd = fsp.open(fileName, 'r')

  // with the open file ...
  return Δfd.then(fd => {
    // read the first bytes into buffer
    const Δread = fsp.read(fd, buff)

    // close the file after reading it
    const Δclosed = Δread.then(_ => fsp.close(fd))

    // after closing, return read buffer
    return Δclosed.then(_ => Δread)
  })
}

//-----------------------------------------------------------------------------
function requirePromised_fs () {
  return {
    open: fsp_open,
    read: fsp_read,
    close: fsp_close,
  }
}

function fsp_open(path, flags) {
  return new Promise(fulfiller)

  function fulfiller(resolve, reject) {
    fs.open(path, flags, function cb (err, fd) {
      if (err) return reject(err)
      resolve(fd)
    })
  }
}

function fsp_read(fd, buffer) {
  return new Promise(fulfiller)

  function fulfiller(resolve, reject) {
    fs.read(fd, buffer, 0, buffer.length, 0, function cb (err, bytesRead, bufferR) {
      if (err) return reject(err)
      resolve(bufferR.slice(0, bytesRead))
    })
  }
}

function fsp_close(fd) {
  return new Promise(fulfiller)

  function fulfiller(resolve, reject) {
    fs.close(fd, function cb (err) {
      if (err) return reject(err)
      resolve(null)
    })
  }
}